[boost] thread lib: thread specific storage problem

From: Chuck Messenger (chuckm_at_[hidden])
Date: 2003-05-24 09:14:08

I've been experimenting with the thread lib.  I found a bug related in 
some way to thread_specific_storage (tss).  In particular, I #ifdef'd 
all the tss stuff out, using native pthread calls instead, and 
everything worked as expected.

So, I went back through, trying to determine what could be going on. 
I'm using tss to store a pointer to a thread-wide structure.

     thread_specific_ptr<mythread*> current_thread;

     mythread *mythread_get() {
         return *(current_thread.get());
     }

     static int init() {
         // Need to initialize thread local storage for main thread

         // XXX I'd like to put something here to ensure that
         // current_thread has been constructed.  But what?

         current_thread.reset(new mythread *);
         *(current_thread.get()) = new mythread;

         return 0;
     }

     static int val = init();

The problem is that I can't be sure, during init(), that current_thread 
has been constructed.  I believe that's at the root of the bug I'm tracking.

By contrast, in my pthread-specific code, I simply put the 
pthread_key_create() call at the start of init(), thus ensuring proper 
initialization order.  But there's no analogous call I can make for a 
thread_specific_ptr -- that call is done during construction time.

So, what to do about it?  Well, one obvious solution is to indirect 
current_thread:

     thread_specific_ptr<mythread*> *current_thread;

then construct current_thread during init().  But that gives me yet one 
more indirection I have to do during mythread_get().  It's already slow 
as it is.

Any suggestions?

Which brings me to another source of slowness: because 
thread_specific_ptr automatically invokes delete on a thread's tss 
pointers when the thread goes away, I can't put a pointer to my real 
object, like this:

     thread_specific_ptr<mythread> current_thread;

because mythread can't be deleted (for arcane reasons I won't get into 
here).  It would be nice if there were a version of thread_specific_ptr 
which didn't delete, e.g.:

     thread_specific_ptr<mythread, non_deleting> current_thread;

Just a suggestion...

     - Chuck